Method, apparatus, computer program and system for measuring oscillatory motion

ABSTRACT

A method, apparatus, computer program, system and device for measuring oscillatory motion comprising: receiving a plurality of signals related to a plurality of measurements of a direction of a vector parameter (a t-1 , a t ); determining a predominant measured direction of the vector parameter (ā t ) based on the plurality of measurements of the direction of the vector parameter; determining an angle of rotation (φ t ) between: a measured direction of the vector parameter (a t ) of one of the plurality of measurements of the direction of the vector parameter, and the predominant measured direction of the vector parameter (ā t ).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. 371 National Stage Application ofInternational Application No. PCT/GB2011/050934, filed May 16, 2011,which claims priority to GB1009379.7, filed on Jun. 4, 2010, the entirecontents of which are incorporated herein by reference.

FIELD OF THE INVENTION

Embodiments of the present invention relate to a method, apparatus,computer program and system for measuring oscillatory motion. Inparticular, they relate to a method, apparatus, computer program andsystem for measuring oscillatory motion related to respiration.

BACKGROUND TO THE INVENTION

The measurement of patient's respiration is useful in the monitoring anddiagnosis of a wide range of respiratory disorders, as well as being auseful broader metric of a patient's condition. Measurements of interestmay include respiratory rate (breaths per minute, BPM), flow, tidalvolume, breathing pattern as well as changes in these over time.

Devices for monitoring breathing typically fall into 3 main categories:

-   -   i) those that observe the movement or tissue composition of the        abdomen and/or chest,    -   ii) those that measure the flow of air, and    -   iii) those that measure concentrations of different gases in the        blood or expired air

Devices of type i) may involve the measuring of: transthoracic andabdominal impedance, a circumference through strain, pressure orfiber-optic gauges embedded in belts and straps, electrical activity ofthe muscles involved in breathing (electromyography). Several of thesedevices require the use of obtrusive constrictive belts placed aroundthe chest and abdomen which may not be conducive to continuous or longterm monitoring, particularly for patients with certain conditions.Other devices require expensive and complex infrastructure around thepatient and/or require a stationary patient to take measurements sincepatient movement disrupts the measurements, which effectively precludesthe ability for home monitoring.

Where a patient requires Oxygen therapy, this can interfere with certaindevices of type ii) which measure nasal gas flow. Also, mouth breathingcannot be measured with devices such as a cannula. Furthermore, a devicemeasuring total breathing gas flow, such as a face mask, may beobtrusive and uncomfortable for patients, thus limiting the ability forcontinuous or long term monitoring of measurements.

Devices of type iii) tend not to give a reliable indication of changesin tidal volume and may react slowly to disruptions in breathing. Suchdevices are often obtrusive and not conducive to continuous or long termor remote home monitoring.

The listing or discussion of a prior-published document or anybackground in this specification should not necessarily be taken as anacknowledgement that the document or background is part of the state ofthe art or is common general knowledge. One or more aspects/embodimentsof the present disclosure may or may not address one or more of thebackground issues.

BRIEF DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

According to various, but not necessarily all, embodiments of theinvention there is provided a method for measuring oscillatory motioncomprising: receiving a plurality of signals related to a plurality ofmeasurements of a direction of a vector parameter (a_(t-1), a_(t));determining a predominant measured direction of the vector parameter(ā_(t)) based on the plurality of measurements of the direction of thevector parameter; determining an angle of rotation (φ_(t)) between: ameasured direction of the vector parameter (a_(t)) of one of theplurality of measurements of the direction of the vector parameter, andthe predominant measured direction of the vector parameter (ā_(t)).

The vector parameter may be a three dimensional vector which relates tofor example: a gravitational field, a magnetic field or an angularorientation measured by a sensor. The sensor provides signals indicativeof a measurement of a direction of the vector parameter relative to thesensor's measurement frame of reference, for example: a direction of theearth's gravitational field (e.g. a direction downwards in the sensor'sframe), a direction of the earth's magnetic field (e.g. a directionnorth in the sensor's frame) or an orientation of the sensor. Movementof the sensor, in particular rotational movement, causes the directionof vector parameter in the sensor's frame to change.

In some embodiments, the sensor is coupled to a body part, for example athorax, torso, chest or ribcage region of a patient at rest whoserespiration is to be monitored, i.e. the sensor is located and attachedto a part of the body which undergoes oscillatory movement duringrespiration. During the respiration process, the thorax region undergoesoscillatory movement as the lungs expand and contract. This movementcauses corresponding movement of the attached sensor which alters themeasured direction of the vector parameter within the frame of thesensor, i.e. it too undergoes an oscillatory movement. Readings of thedirection of the vector parameter are taken. A predominant or averagedirection of the measured direction of the vector parameter isdetermined from a plurality of measurements over a period of time. Thecalculated predominant direction provides a base of reference for use indetermining an angle of rotation due to the movement. The angle ofrotation is defined as the angle between a measured direction of thevector parameter and the predominant measured direction of the vectorparameter.

Embodiments provide a new metric for measuring oscillatory movementwhich is directly related to a physical parameter, e.g. rotation of anobject during oscillatory motion or rotation of a patient's chest wallduring respiration. Advantageously, the metric is an angular value thatis determined relative to a reference direction which is determined fromthe measurements themselves, as opposed to being an absolute angularvalue that is relative to some constant or fixed reference direction.Thus, were there to be a large scale movement, unrelated to theoscillatory movement, which shifts the reference direction within thesensor's frame, the reference direction can be re-determined.

Various but not necessarily all embodiments provide the advantage that ameasurement value, the angle of rotation, is determined which directlyrelates to a physical measurement, namely the angular motion induced byan oscillatory motion. In the case of respiration measurement, e.g.measurements related to the act of breathing where measurements aretaken of the torso, the angular value directly relates to angularmovement of torso during respiration with respect to a determinedreference direction which corresponds to an average/central direction inthe oscillating motion. The angle of rotation is determined relative tothe determined reference direction, and so is independent of the initialorientation of a patient and the sensor device which captures themeasurements. Thus, precise and careful initial placement of the sensoronto a patient is not required.

Various but not necessarily all embodiments may improve the ability toprovide continuous unobtrusive respiratory monitoring. Embodiments mayprovide the ability to adapt and adjust to patient re-orientation orlarge scale movement by re-setting the reference direction against whichthe angle of rotation is determined. Various embodiments do not requirerestraining apparatus or complex and bulky infrastructure nor theinvolvement or medical practitioners and thus can enable low cost andlong term remote respiratory monitoring.

According to various, but not necessarily all, embodiments of theinvention there is provided a method which further includes:

determining a predominant axis of rotation about which the direction ofthe measured vector parameter rotates and further determining the angleof rotation based on rotation about the predominant axis of rotation.

The resultant determined angle relates to an angle of rotation about thepredominant axis of rotation between: the measured direction of thevector parameter and the predominant measured direction of the vectorparameter. In effect, such embodiments provide for the determination ofan angle which is a projection of the angle of rotation onto a planeperpendicular to the predominant axis of rotation.

By only considering one dimensional angular motion projected onto asurface such as a plane perpendicular to the predominant axis ofrotation, this provides the advantage that the quality and accuracy ofthe resultant determined angle is improved since any extraneousmeasurement signals or noise, e.g. due to movements/rotations in otherplanes which are not due to respiratory related movement are ignored. Invarious but not all embodiments, the determined angle of rotation isonly concerned with rotations about the predominant axis of rotation.Thus, such embodiments provide an enhanced metric for measuringrespiration.

According to various, but not necessarily all, embodiments of theinvention there is provided a method which further includes:

determining a rotation angle (θ) between: a measured direction of thevector parameter (a_(t-1)) and a sequential measured direction of thevector parameter (a_(t)).

If the rotation angle (θ) is found to be greater than or equal to athreshold value, this may be indicative of substantial movement whichwould affect the direction of the predominant direction of the vectorparameter as well as the predominant axis of rotation thereby disruptingthe determined angle of rotation. Such substantial or “macro” movementof the patient relates to movement which is not related to the relative“micro” movement of the oscillating movement to be measured. Upondetection of exceeding the threshold, the predominant direction of thevector parameter and predominant axis of rotation are re-determined.Such features provide the ability to distinguish valid measurementsignals from invalid measurement signals and enable the classificationof periods between the threshold being exceeded where the patient isstatic and measurements can be taken which are not overwhelmed bysubstantial movement. Advantageously, this enables embodiments to adoptand adjust to substantial movements which would otherwise cause a shiftin the reference direction and lead to inaccurate measurements.

According to various, but not necessarily all, embodiments of theinvention there is provided a method which further includes:

determining an angular velocity (ω) based on angle of rotation (φ).

Advantageously, the inventors have found that the value of angularvelocity closely corresponds to respiratory flow rate. Thus,advantageously, embodiments can be used to provide a metric that relatesto respiratory flow rate.

According to various, but not necessarily all, embodiments of theinvention there is provided an apparatus for measuring oscillatorymotion comprising: a memory storing computer program instructions; aprocessor configured to execute the computer program instructions tocause the apparatus at least to perform the above methods.

According to various, but not necessarily all, embodiments of theinvention there is provided a computer program that, when run on acomputer, performs the above methods.

According to various, but not necessarily all, embodiments of theinvention there is provided an apparatus for measuring oscillatorymotion comprising: an input interface arranged to receive a plurality ofsignals related to a plurality of measurements of a direction of avector parameter; a controller arranged to determine a predominantdirection of the vector parameter based on the plurality of measurementsof the direction of the vector parameter; and the controller furtherarranged to determine an angle of rotation between: a measured directionof the vector parameter of one of the plurality of measurements of thedirection of the vector parameter and the predominant measured directionof the vector parameter.

According to various, but not necessarily all, embodiments of theinvention there is provided a system or device for measuring oscillatorymotion comprising: means for measuring a direction of a vectorparameter; means for generating a signal related to the direction of avector parameter; means for transmitting the signal; and the aboveapparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of various examples of embodiments of thepresent invention reference will now be made by way of example only tothe accompanying drawings in which:

FIG. 1 schematically illustrates a flow diagram of a method formeasuring oscillatory motion;

FIG. 2 schematically illustrates a flow diagram of another method formeasuring oscillatory motion;

FIG. 3 schematically illustrates an example of a sensor for measuringoscillatory motion;

FIG. 4 schematically illustrates a flow diagram of another method formeasuring oscillatory motion;

FIG. 5 illustrates a graph of angular rate and respiratory rate againsttime;

FIG. 6 schematically illustrates an example of an apparatus formeasuring oscillatory motion;

FIG. 7 schematically illustrates an example of a system for measuringoscillatory motion;

FIG. 8 schematically illustrates an example of a device for measuringoscillatory motion;

FIG. 9 illustrates a graph showing a comparison between a plot ofangular rotation rates measured by an accelerometer based sensoraccording to an embodiment of the present invention and a plot ofangular rotation rates measured by a gyroscopic sensor against time;

FIG. 10 illustrates a graph showing a comparison between rotation ratesmeasured by an accelerometer based sensor according to an embodiment ofthe present invention and nasal cannula pressure against time;

FIG. 11A illustrates a graph showing a tri-axial accelerometer signalagainst time;

FIG. 11B illustrates a graph showing the angle changes betweenconsecutive acceleration vectors of FIG. 11A against time;

FIG. 11C illustrates a graph showing angular rates calculated from theaccelerometer signal of FIG. 11A against time;

FIG. 11D illustrates a graph showing correlation coefficients betweenangular rates of FIG. 11C and cannula pressure against time;

FIG. 11E illustrates a graph showing respiratory rates calculated fromthe angular rates of FIG. 11C and respiratory rates calculated fromcannula pressure derived data against time; and

FIG. 12 schematically illustrates a flow diagram of another method formeasuring oscillatory motion;

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

FIG. 1 schematically illustrates a flow diagram of a method 100 formeasuring oscillatory motion. In block 101, a plurality of measurementsof a direction of a vector parameter at given points in time (a_(t-1),a_(t)) are received.

The measurements of the vector parameter are captured by a sensor whichis fixedly attached or physically coupled to the object whoseoscillatory movement is to be measured, such that the sensor is kept ina stable and steady position with respect to the object so that movementof the object causes corresponding movement of the sensor. Themeasurements are captured at a predetermined frequency. Preferably, thefrequency is set such that a plurality of measurements is captured overthe duration of a single cycle of the oscillatory motion.

In some embodiments, the sensor is fixedly attached directly orindirectly to a body part, for example a thorax, torso, chest, abdomenor ribcage region of a patient whose respiration is to be monitored, ora garment of the patient. The sensor is located and attached to a partof the body which undergoes oscillatory movement during respiration. Thesensor may be directly or indirectly physically coupled to the body partby any suitable means such as, for example, via fixing/attaching meansor adhesion. Preferably, the sensor device is affixed to the lowerboundary of a patient's false ribs, on the mid clavicular line tomaximize the amplitude of the measured oscillatory motion. Also,simultaneous placement of another sensor device on the thoracic cage andthe abdomen would allow the capture of different stages andcharacteristics of breathing.

The vector parameter may be a three dimensional vector which relates to:a gravitational field, a magnetic field or an angular orientationmeasured by a sensor. The sensor provides signals indicative of ameasurement of a direction of the vector parameter in the sensor'smeasurement frame of reference, for example: a direction of the earth'sgravitational field (e.g. a direction downwards in the sensor's frame),a direction of the earth's magnetic field (e.g. a direction north in thesensor's frame) or an orientation of the sensor. Movement of the sensor,in particular oscillatory rotational movement, causes the direction ofvector parameter in the sensor's frame to change.

Where the sensor is an accelerometer, due to the minor linearaccelerations involved in the motions due to breathing, the measuredacceleration vector parameter will be close to the acceleration due togravity g whilst the patient is at rest and breathing. As the sensorrotates, the gravity vector would rotate in the co-ordinate frame of thesensor. The plurality of measurements enables the gravity vector'sdirection with respect to the sensor's frame of reference to be trackedduring the oscillatory movement induced by breathing.

In block 102, a predominant direction of the vector parameter (ā_(t))over a pre-determined period of time (W) is determined. This correspondsto an average direction of the vector parameter in the sensor's frameover the pre-determined period of time. Preferably, the time period overwhich measurements are taken is greater than or equal to the duration ofone cycle of the oscillatory motion, e.g. the duration of a breath or arespiration cycle. This enables measurements to be captured over atleast one entire cycle of oscillatory motion.

The predominant direction of the vector parameter ā_(t) can becalculated as:

${\overset{\_}{a}}_{t} = {{normalise}\left( {\sum\limits_{i = {- \frac{W}{2}}}^{\frac{W}{2}}a_{t + i}} \right)}$

In block 103, an angle of rotation (φ_(t)) is determined. This angle isdefined as the angle between:

one of the measured directions of the vector parameter (a_(t)), and

the predominant measured direction of the vector parameter (ā_(t)).

The angle of rotation φ_(t) can be calculated as:

φ_(t)=cos⁻¹(a _(t) ·ā _(t))

An angular rotation rate or angular velocity ω_(t) of the angle ofrotation φ_(t) can be determined. This can be calculated by taking thederivative ω_(t) of φ_(t) i.e.

$\omega_{t} = \frac{\varphi}{t}$

Where the angular velocities are related to respiration, otherrespiration parameters can be determined from the angular velocities byapplying various different algorithms to determine: breathing rates,breathing variability and changes in chest expansion.

FIG. 2 schematically illustrates a flow diagram of another method 200for measuring oscillatory motion which involves the determination of theaxes of rotation of the vector parameter.

As the sensor rotates, the measured vector parameter (e.g. the measuredgravity vector, magnetic vector or orientation vector) rotates in theco-ordinate frame of the sensor. However, the axis of this rotation maybe arbitrarily orientated in the sensor frame, and may change due todifferences in the way the patient breathes at different times. Theorientation of the axis can also change following a large scalemovement, i.e. unrelated to the oscillatory movement, such as a changein the overall orientation of the patient. In order to reduce the affectof noise in measured signals, it is advantageous only to considerrotations about the axis of rotation. Thus, it is desirable to determinethe axis of rotation and track it as it changes.

In block 201, a plurality of measurements of directions of a vectorparameter (a_(t-1), a_(t)) are received. The measurements comprise pairsof sequentially measured measurements of the vector parameter.

Similarly to block 102, in block 202 a predominant direction of thevector parameter (ā_(t)) is determined.

In block 203, an axis of rotation (r_(t)) between each pair ofsequential measurements (a_(t-1), a_(t)) is determined. The axis ofrotation it can be calculated as:

r _(t) =a _(t) ×a _(t-1)

For an oscillatory rotation, r_(t) will invert when the direction ofrotation reverses. Accordingly, the axis direction is normalized(r_(t)′) so as to be within a chosen hemisphere by comparison to areference axis (r_(ref)). The normalized axis of rotation directionr_(t)′ can be calculated as:

$r_{t}^{\prime} = \left\{ \begin{matrix}{r_{t},} & {{r_{t} \cdot r_{ref}} \geq 0} \\{{- r_{t}},} & {{r_{t} \cdot r_{ref}} < 0}\end{matrix} \right.$

The value of r_(ref) can be selected manually, or chosen automaticallybased on initial observations, such as by performing principalcomponents analysis on the values of r_(t).

In block 204, a predominant axis of rotation ( r _(t)) is determined.This corresponds to an average axis of the axes of rotation determinedin block 203. Again, this is determined over a pre-determined period oftime (W).

The predominant axis of rotation r _(t) could be calculated by taking anaverage of all the normalized axes of rotation r_(t)′ over apre-determined period of time (W), i.e.:

${\overset{\_}{r}}_{t} = \left( {\sum\limits_{i = {- \frac{W}{2}}}^{\frac{W}{2}}r_{t + i}^{\prime}} \right)$

Alternatively, in order to reduce the effect of noise on thedetermination of the mean r _(t), each normalized axis of rotationdirection r_(t)′ could be suitably weighted, for example by an anglechange φ_(t) associated with each pair of sequentially measuredmeasurements of the vector parameter (a_(t-1), a_(t)). Also, to give agreater weight to estimates closer in time to t, a Hamming windowfunction H(n) can be used, i.e.:

${\overset{\_}{r}}_{t} = {{normalise}\left( {\sum\limits_{i = {- \frac{W}{2}}}^{\frac{W}{2}}{{H(i)}\theta_{t + i}r_{t + i}^{\prime}}} \right)}$

where:

θ_(t)=cos⁻¹(a _(t) ·a _(t-1))

In block 205, an angle of rotation φ_(t) is determined which is definedas an angle of rotation about the predominant axis of rotation ( r _(t))between:

a measured direction of the vector parameter (a_(t)) and

the predominant direction of the vector parameter (ā_(t))

The angle of rotation φ_(t) can be calculated as:

φ_(t)=sin⁻¹((ā _(t) × r _(t))·a _(t))

One can consider block 205 as further determining the angle of rotationwhich was determined in FIG. 1's block 103 by a projection of block103's angle of rotation onto a surface. In block 205, the surfacecorresponds to a plane, defined as a plane perpendicular to thepredominant axis of rotation r _(t). However, it would be possible forother projection surfaces to be considered, such as for example, aprojection of block 103's angle of rotation onto a surface of anellipsoid. To effect this, the initial measurements of directions of avector parameter (a_(t-1), a_(t)) could be appropriately modified byapplication of a suitable function f(a).

In block 206, the angular rotation rate or angular velocity ω_(t) of theangle of rotation φ_(t) is determined. This can be calculated by takingthe derivative φ_(t) of, i.e.

$\omega_{t} = \frac{\varphi}{t}$

FIG. 3 schematically illustrates an example of a sensor device 300 formeasuring oscillatory motion. In particular, the figure provides arepresentation of the various axes and vectors used above.

The sensor device comprises sensors that can give an indication ofmovement of the device. For example:

an accelerometer that can detect changes in the measured direction ofthe earth's gravitational field (such as a Freescale® MMA7260QTtri-axial accelerometer)

a magnetometer that can detect changes in the measured direction of theearth's magnetic field (such as a Honeywell® HMC1052 twin axismagnetometer), or

a gyroscope that can detect changes in the sensor's orientation (such asAnalog Devices® ADXRS300 rate gyroscopes).

The sensor device is configured to provide measurements that enable avector quantity, such as three dimensional representation of theorientation of the sensor device, to be determined.

The sensor device's co-ordinate plane is shown as three orthogonal axes:x, y and z. The sensor device comprises a tri-axial sensor able tomeasure the vector parameter in each of three orthogonal axes ordimensions. The use of a tri-axial device allows inclination changes tobe measured regardless of orientation. If the sensor device is placed onan object which undergoes oscillatory rotation over time, then themeasured directions of a vector parameter within the sensor device'sframe of measurement (a_(t-1), a_(t)) would alter as indicated. Thepredominant direction of the vector parameter ā_(t) relates to anaverage direction of the measured directions of the vector parameterover time.

The axis of rotation r_(t) relates to an axis about which a firstmeasured direction a_(t-1) of the vector parameter rotates with respectto a second measured direction of the vector parameter a_(t). The anglethrough which the direction of the vector rotates is the angle changeθ_(t). The axis of rotation is perpendicular to each of the first andsecond measured vector parameters.

The predominant axis of rotation r _(t) relates to an average of theaxes of rotation. The angle of rotation (φ_(t)) relates to the anglebetween: one of the measured directions of the vector parameter (a_(t)),and the predominant measured direction of the vector parameter (ā_(t)).In the method of FIG. 2, the angle is additionally defined as beingbased on rotation about the predominant axis of rotation r _(t).

In various but not necessarily all embodiments, accelerometers areemployed to measure a vector parameter. It may seem counterintuitive toemploy an accelerometer to measure an angle and angular rate, whengyroscopic microelectromechanical systems (MEMS) exist which can do thisdirectly. However, advantageously, accelerometer-based sensors have alower power consumption than gyroscopic based sensors. For example,current tri-axial MEMS accelerometers can have active currentconsumptions of as little as 40 μA, permitting very long term monitoringwith small battery-powered devices, whereas, integrated tri-axial MEMSgyroscopes have a current consumptions of the order of 10 mA.

When a patient undergoes large scale movement, e.g. which results in achange of body position, the accelerometer based measurement methodduring such a period of motion is likely infeasible since the magnitudeof the movement-induced signals due to patient movement, i.e. movementunrelated to the oscillatory movement under investigation, vastlyexceeds the oscillatory movement due to breathing. Thus, it is desirableto detect such movements so that measurements captured around the timeof such movement are ignored as they would not reliably relate just tothe oscillatory motion under investigation. Furthermore, following apatient re-orientation, the predominant direction of the vector and thepredominant axis of rotation are likely to have altered and thus wouldneed to be re-calculated.

FIG. 4 schematically illustrates a flow diagram of another method 400for measuring oscillatory motion which addresses such issues andprovides a method to distinguish periods when the patient is at rest andmeasurements can reliably be taken and periods when the patient ismoving/re-orientating, during which measurements may be disregarded.

The method 400 follows on from the previously described methods of FIG.2 including block 201 in which a plurality of measurements of directionsof a vector parameter (a_(t-1), a_(t)) are received.

In block 401, the method 400 includes the determination of a change inthe pair of sequentially measured measurements of the vector parameter(a_(t-1), a_(t)). The change could be a change in magnitude or a changein direction of the measured vector parameter between sequentialmeasurements. Block 401 shows the determination of an angle change θ_(t)associated with each pair of sequentially measured measurements of thevector parameter (a_(t-1), a_(t)), e.g.

θ_(t)=cos⁻¹(a _(t) ·a _(t-1)).

In block 402, the change in the measured vector parameter is compared toa threshold value, e.g. θ_(T). The threshold value is set at apre-determined value that exceeds the amount of change one would expectto be feasible for the oscillatory motion under investigation. Forexample, with regards to measuring respiration, a sudden increase in themeasured acceleration between two consecutive readings would likelyindicate macro scale movement of the patient, i.e. the sensor would havepicked up movement of the patient which is not related to breathing.Were the sensor to use measurements of movements unrelated to breathing,this would lead to erroneous measurements. Thus, the features of FIG. 4provide a method to detect such large scale movements unrelated to theoscillatory movements to be measured. Measurements captured which relateto such large scale movements could then be disregarded. Furthermore,following such movement, it is likely the orientation of the sensor mayhave substantially changed. This would alter the predominant measureddirection of the vector parameter (ā_(t)) in the sensor's frame ofreference. Also, it would alter the predominant axis of rotation ( r_(t)) in the sensor's frame of reference. Thus, these quantities wouldneed to be re-determined.

If the determined change in the sequential measurements of the vectorparameter exceed a threshold level, indicated by reference 403, thenfurther measurements of a direction of the vector parameter are obtainedb_(t-1), b_(t) (block 405) which are used to re-calculate thepredominant measured direction of the vector parameter b (block 405) anda predominant axis of rotation r _(bt) (block 406) which are used todetermine an angle of rotation φ_(bt) (block 407) and an angularvelocity ω_(bt) (block 408).

Thus, the determined values of the angle of rotation and the angularvelocity are based on measurements captured during a time window duringwhich no substantial movements took place, which would overwhelm themeasurements of movement just related to breathing.

A range of measured measurements could be determined and used to providean indication whether or not the measurements recorded are sub optimal.If the determined range of measurements is below a threshold level, thismay indicate that the sensor is not adequately measuring the oscillatorymovement. For example, the sensor might be inappropriately positioned orsub-optimally located on a body part that does not undergo therespiration related movement. Alternatively, the sensor may have becomedetached from the body part and thus, being no longer fixedly attachedto it, does not follow its movements.

A degree of confidence of the angle of rotations and angular velocitiescan be determined based on a weighted sum of different measures of thequality of the measurements, for example: the amplitude of themeasurements of the vector parameter, the noise level outside thefrequencies relevant for the oscillatory motion, the degree of signalfound to be outside of the plane of rotation and the angular consistencyof the angular rotation rate.

FIG. 5 illustrates a graph 500 of angular rate and respiratory rate 501against time based upon measurements recorded with embodiments of thepresent invention. The graph also shows a respiratory rate 502 againsttime.

In order to provide a method of determining a respiratory rate, i.e. thenumber of breaths per minute, from measured angular velocities, a firstband (B_(H)) of range of angular velocity values is defined where theangular velocity value is greater that a first threshold value (ω_(H)).A second band (B_(L)) of range of angular velocity values is alsodefined where the angular velocity value is less than a second thresholdvalue (ω_(L)). A third band (B_(M)) of range of angular velocity valuesare also defined where the angular velocity value is between the firstand second threshold values (φ_(H), ω_(L)).

The values of sequential angular velocity measurements are monitored anda registration of a count corresponding to the completion of one cycleof the oscillatory motion, e.g. one breath, is recorded each time thevalues of angular velocity transition through four bands, e.g. T₁ fromB_(M) to B_(H), T₂ from B_(H) to B_(M), T₃ from B_(M) to B_(L) and T₄from B_(L) to B_(M). The respiration rate is then determined based onthe number of counts within a measurement time period.

Prior approaches to determining a respiratory rate which involvespectral analysis of waveforms had difficulty detecting individualbreaths where a patient's breathing pattern is irregular.Advantageously, the above state transition approach provides the abilityto identify individual breaths, including breaths of irregularfrequency. Also, it reduces problems of multiple breaths beingregistered from noisy signals as might occur with a single thresholdmethod were repeated crossings through a single threshold would lead toerroneous results.

FIG. 6 schematically illustrates an example of an apparatus 600 formeasuring oscillatory motion.

The apparatus 600 comprises a controller such as a processor or digitalsignal processor 604; and a memory 601 including computer program 602comprising program code 603. The memory 601 and the computer programcode 603 are configured to, with the processor 604, cause the apparatus600 at least to perform the methods described above with reference toFIGS. 1, 2 and 4.

The blocks illustrated in the FIGS. 1, 2 and 4 may represent steps in amethod and/or sections of code in the computer program 602. Theillustration of a particular order to the blocks does not necessarilyimply that there is a required or preferred order for the blocks and theorder and arrangement of the block may be varied. Furthermore, it may bepossible for some steps to be omitted.

The processor 604 is configured to read from and write to the memory601. The processor may also comprise an output interface 606 via whichdata and/or commands are output by the processor and an input interface605 via which data and/or commands are input to the processor.

The memory 601 stores a computer program 602 comprising computer programinstructions 602 that control the operation of the apparatus 600 whenloaded into the processor 604. The processor 604, by reading the memory601, is able to load and execute the computer program 602. The computerprogram instructions 603 provide the logic and routines that enable theapparatus to perform the methods illustrated in FIGS. 1, 2 and 4 anddiscussed above.

The computer program 602 may arrive at the apparatus 600 via anysuitable delivery mechanism. The delivery mechanism may be, for example,a computer-readable storage medium, a computer program product, a memorydevice, a record medium such as a compact disc read-only memory ordigital versatile disc, an article of manufacture that tangibly embodiesthe computer program 602. The delivery mechanism may be a signalconfigured to reliably transfer the computer program 602.

Implementation of the controller 604 can be in hardware alone (acircuit, a processor . . . ), have certain aspects in software includingfirmware alone or can be a combination of hardware and software(including firmware).

The controller 604 may be implemented using instructions 603 that enablehardware functionality, for example, by using executable computerprogram instructions in a general-purpose or special-purpose processorthat may be stored on a computer readable storage medium (disk, memoryetc) or carried by a signal carrier to be executed by such a processor.

FIG. 7 schematically illustrates an example of a system 700 formeasuring oscillatory motion. The system comprises means for measuring adirection of a vector parameter such as sensor devices 300 and 701 whichare coupled to a means for generating a signal related to the directionof a vector parameter, such as a controller 702.

Preferably each sensor device is capable of measuring a vector parameterin 3 dimensions. Furthermore, where two or more sensor devices areemployed, it is preferable that each measures a different vectorparameter. For example, sensor device 300 is an accelerometer basedsensor device for measuring gravitational fields and sensor device 701is a magnetometer based sensor device for measuring magnetic fields.Where solely a single type of sensor device is employed, e.g. justaccelerometer based, any component of rotations that lie in the verticalaxis, i.e. aligned with direction of gravitational field, would not bedetected or measured by the accelerometer. However, by additionallyincluding another type of sensor such as a magnetometer or gyroscope,such rotations aligned with direction of gravitational field would bedetectable and measureable. Similarly, where sensor is just magnetometerbased, any component of rotations that lies along an axis aligned withdirection of the earth's magnetic field would not be detected by themagnetometer. Additionally including an accelerometer or gyroscope wouldenable such rotations to be detected and measured.

The controller 702 is coupled to a means for transmitting the signal,such as a transmitter 703, to apparatus 600 as shown in FIG. 6. Thetransmission 706 could be effected via wired or preferably wirelesscommunication to facilitate the remote capture and monitoring ofmeasurements.

Optionally, storage 704 is provided to store the measurements so thatmeasurements need not be constantly sent/transmitted as soon as they aremeasured. In some embodiments, the sensors, controller, transmitter andoptional storage are all housed within housing 705. Preferably, thehousing and internal components are dimensioned as small as possible sothat the device is minimally obtrusive when attached to a patient. Suchsmall housings may be attached to a patient with standard medical tape.

FIG. 8 schematically illustrates an example of a device 800 formeasuring oscillatory motion. The device 800 effectively comprises theapparatus for processing the measurements signals, as discussed withregards to FIG. 6, and also sensor device 300 (and optionally 701) formeasuring and generating the measurement signals all housed withinhousing 801.

The device 800 comprises a controller 604, memory 601, computer program602 and program code 603. Sensor devices 300 and 701 capturemeasurements of different vector parameters which are received by theinput interface 605 of the processor 605. The memory 601 and thecomputer program code 603 are configured to, with the processor 604,cause the device 801 at least to perform the methods described abovewith reference to FIGS. 1, 2 and 4. The determined angles of rotationsφ_(t) and angular velocities ω_(t) are an output from interface 606 totransmitter 703 for wired or wireless transmission 803 to a basestation, computing device or server 802. Preferably, the device 800comprises its own on board power supply, such as batteries and theability to re-charge the batteries (not shown).

Optionally, two or more devices are fixedly attached to a patient whoserespiration is to be measured. At least one of the devices is fixedlyattached to a body part that undergoes oscillatory motion due torespiration. At least one of the devices is fixedly attached to a bodypart that does not undergo oscillatory motion due to respiration.Measurements from the two or more devices can then be combined to removemeasurement components not related to the oscillatory motion underinvestigation. This enables a more precise determination of componentsof the measurements that are solely related to respiration.

FIG. 9 illustrates a graph showing a comparison between angular rotationrates ω_(t) measured by each of an accelerometer based sensor accordingto an embodiment of the present invention and a gyroscopic sensoragainst time. As can be seen, the angular rates obtained from theaccelerometer, having been appropriately scaled, provide a good match toangular rates obtained via a gyroscopic device. The graph shows a rangeof different breathing rates and depths which are visible from eachplot.

FIG. 10 illustrates a graph showing a comparison between rotation ratesω_(t) measured by an accelerometer based sensor according to anembodiment of the present invention and nasal cannula pressure againsttime.

This graph illustrates the correspondence of angular rotation ratesω_(t) and a nasal cannula pressure measured by a pressure transducer(Respironics® PTAF2) connected to a nasal cannula. It is known that thecannula pressure is proportional to airflow rate for nasal breathing.Thus, the correlation shown in this graph also indicates the correlationbetween the angular rotation rates ω_(t) and flow rates. Furthermore,this is indicative of a correlation between the angular rotation ratesω_(t) and changes in the volume of the chest during breathing.

This correlation is important since the shape of a flow rate waveformprovides significant additional information for diagnosis of respiratoryproblems that is not present in a simple measurement of the respiratoryrate. Analysis of different inspiratory flow shapes suggests the shapeof breaths can be a useful indicator of different respiratoryconditions. It has also been shown that the shape can be used toidentify obstructions.

An evaluation of an embodiment of the method, apparatus and system ofthe present invention was carried out in a 6.9 hour overnight capture ofa post-operative patient. The key data from this evaluation is plottedin FIGS. 11A to E.

FIG. 11A illustrates a graph showing a tri-axial accelerometer signalagainst time in each of an: x axis (the middle plot) y axis (the lowerplot) and z axis (the upper plot). Several large changes of patientorientation are visible as step changes in the acceleration on eachaxis.

FIG. 11B illustrates a graph showing the angle changes θ_(t) betweenconsecutive acceleration vectors of FIG. 11A against time. There areclear spikes in this graph which correspond to major orientationchanges, along with smaller peaks indicating lesser movements. Thethreshold value used for movement detection, θ_(T), is also shown. Athreshold of 5 milliradians per 1/16 s sample interval was used. Thechoice of threshold represents a tradeoff between accuracy and coverage.Lower thresholds increase the sensitivity to movement, discarding moreof the dataset but achieving higher correlation to the cannula data forthe periods retained.

FIG. 11C illustrates a graph showing angular rates calculated from theaccelerometer signal of FIG. 11A against time. Individual breaths arenot visible at this horizontal scale, but the plot illustrates thedetected static periods over which the method algorithm was executed incontrast to the blank periods of no measurement caused bymovement/re-orientation of the patient, i.e. where angle changes θ_(t)exceed the threshold value used for movement detection, θ_(T). Changesin scale of the signal in different orientations, due to variation inthe vertical component of rotations is also noticeable.

FIG. 11D illustrates a graph showing correlation coefficients betweenangular rates of FIG. 11C and cannula pressure against time for each ofthe static periods. The mean is 0.9148.

FIG. 11E illustrates a graph showing respiratory rates calculated fromthe angular rates of FIG. 11C and respiratory rates calculated fromcannula pressure derived data against time for each of the staticperiods. The accelerometer-based rate is only defined for timessurrounded by a full window of angular rate information, and thereforecovers a further reduced set of time segments.

In several of the examples discussed above, the values of angles ofrotation φ_(t) or the angular velocities ω_(t) have been used to providerespiration related signals, for example: to identify angular rotationand rotation rate of a patient's torso, to monitor breathing andrespiratory rates. However, the values of the angles of rotation φ_(t)or the angular velocities ω_(t) can also be used to determine additionalpatient movement related parameters, such as patient speech as well asmovement of the patient such as walking, running or climbing stairs.

The detection and identification of such additional movements can beused in determining periods of activity of the patient (whenmeasurements of respiration would not be possible as the respirationmovement component of the measurement signals would be masked or swampedby the additional movement) and static periods of rest of the patient(where respiration measurements would be feasible). Additionally, thedetection and identification of such additional movements may providecontext to the monitored values of φ_(t) and ωt by indicating, forexample, when a patient was sleeping, walking, talking or running.Furthermore, an intensity of the level of additional movement could begauged which would enable identification, for example, of periods ofintensive exercise.

To further improve the accuracy of the respiration measurements,measurements captured during detected periods of activity or additionalmotion (i.e. motion over and above that of just respiratory relatedmotion) could be disregarded and only utilized during a determinedperiod of inactivity of the patient.

FIG. 12 schematically illustrates a flow diagram of another method 120for measuring oscillatory motion.

In block 121 a series of values of angles of rotation φ_(t) or angularvelocities ω_(t) are determined as previously discussed above.

In block 122, a pattern recognition algorithm is applied to the seriesof values of angles of rotation φ_(t) or the angular velocities ω_(t) todetect patterns, shapes and frequencies in the respective φ_(t) ω_(t)waveforms. Such patterns, shapes and frequencies of the wave form may beindicative of certain movements, such as respiratory related movementswhen the patient is static as discussed above, but also other nonrespiratory related movements such as a patient: speaking, walking orrunning.

In block 123, an identification of the movement is made based on theresults of the pattern recognition. For example, the movement would beidentified as any of, e.g.: breathing, speaking or patient movement suchas walking, or running.

In block 124, a control signal is generated in response to theindentified movement. For example, if the movement were identified asjust breathing, then control signal could cause the use of themeasurements to determine respiration signals. If the movement wereidentified as some other movement (i.e. not related just to breathing)such as speaking, walking, or running then the control signal could beto disregard measurements for the use of determining respirationsignals.

Although various embodiments of the present invention have beendescribed above with reference to various examples, it should beappreciated that modifications to the examples given can be made withoutdeparting from the scope of the invention as claimed. Features describedin the preceding description may be used in combinations other than thecombinations explicitly described.

Although functions have been described with reference to certainfeatures, those functions may be performable by other features whetherdescribed or not. Although features have been described with referenceto certain embodiments, those features may also be present in otherembodiments whether described or not.

Whilst endeavouring in the foregoing specification to draw attention tothose features of the invention believed to be of particular importanceit should be understood that the Applicant claims protection in respectof any patentable feature or combination of features hereinbeforereferred to and/or shown in the drawings whether or not particularemphasis has been placed thereon.

We claim:
 1. A method for measuring oscillatory motion comprising:receiving a plurality of signals related to a plurality of measurementsof a direction of a vector parameter; determining a predominant measureddirection of the vector parameter based on the plurality ofmeasurements; and determining an angle of rotation between: a measureddirection of the vector parameter of one of the plurality ofmeasurements, and the predominant measured direction of the vectorparameter.
 2. A method as claimed in any previous claim, wherein theplurality of measurements comprises a plurality of pairs of sequentialmeasurements of a direction of the vector parameter; the method furthercomprising: determining an of axis of rotation for each pair ofsequential measurements, wherein the axis of rotation is defined as anaxis about which the measured direction of the vector parameter rotatesbetween a pair of sequential measurements of the direction of the vectorparameter; determining a predominant axis of rotation based on thedetermined axes of rotation; and wherein the angle of rotation isfurther determined based on rotation about the predominant axis ofrotation.
 3. A method as claimed in any previous claim, furthercomprising determining an angular velocity based on the angle ofrotation.
 4. A method as claimed in any previous claim, wherein thesignals related to a measurement of a direction of the vector parameterare received from at least one sensor coupled to an object whoseoscillatory motion is to be measured.
 5. A method as claimed in claim 4,wherein the object is a body part.
 6. A method as claimed in anyprevious claim, wherein the vector parameter comprises at least one of:gravitational field, magnetic field and angular orientation.
 7. A methodas claimed in any previous claim, wherein each signal related to ameasurement of a direction of the vector parameter comprises arepresentation of the direction of the vector parameter in threeorthogonal axes.
 8. A method as claimed in any previous claim, whereinat least one of: receiving the plurality of signals related to theplurality of measurements of the direction of the vector parameter,determining the predominant direction of the measured vector parameter,or determining the predominant axis of rotation; occurs over apredetermined period of time.
 9. A method as claimed in any previousclaim, wherein the predetermined period of time is greater than or equalto the duration of a cycle of the oscillatory motion.
 10. A method asclaimed in any previous claim, further comprising determining a changebetween: a measurement of the vector parameter and a sequentialmeasurement of the vector parameter.
 11. A method as claimed in previousclaim 10, further comprising determining if the change is greater than athreshold value; and if the change is greater than a threshold value:receiving a further plurality of signals related to a plurality ofmeasurements of a direction of the vector parameter; re-determining thepredominant direction of the vector parameter based on the furtherplurality of measurements; and determining an angle of rotation between:a measured direction of the vector parameter of one of the furtherplurality of measurements of the direction of the vector parameter, andthe re-determined predominant direction of the vector parameter.
 12. Amethod as claimed in any previous claim, further comprising: determininga plurality of angular velocities; monitoring values of angularvelocities; defining a first band of range of angular velocity valueswhere an angular velocity value is greater that a first threshold value;defining a second band of range of angular velocity values where anangular velocity value is less than a second threshold value; defining athird band of range of angular velocity values where an angular velocityvalue is between the first and second threshold values; and registeringa count of a cycle of the oscillatory motion upon determining monitoredvalues of angular velocity undergoing 4 band transitions.
 13. A methodas claimed in any previous claim, further comprising: receiving aplurality of signals related to a plurality of measurements of adirection of second vector parameter; determining a predominantdirection of the second vector parameter based on the plurality ofmeasurements of the direction of the second vector parameter; anddetermining an angle of rotation between: a measured direction of thesecond vector parameter and the predominant direction of the secondvector parameter.
 14. A method as claimed in previous claim 13, whereinthe second vector parameter differs from the vector parameter.
 15. Amethod as claimed in any previous claim, further comprising determiningan angle of projection, wherein the angle of projection is defined as aprojection of the angle of rotation onto a surface.
 16. A method asclaimed in previous claim 15, wherein the surface comprises one of aplane or at least part of an ellipsoid.
 17. A method as claimed in claim3, further comprising: determining a series of values of angularvelocities or angles of rotation; and applying a pattern recognitionalgorithm to the series of values of angular velocities or angles ofrotation.
 18. A method as claimed in claim 17, further comprisingapplying a pattern recognition algorithm to the series of values ofangular velocities or angles of rotation in order to identify a motion.19. A method as claimed in claim 18, further comprising: generating acontrol signal in response to identification of the motion.
 20. Anapparatus for measuring oscillatory motion comprising: a memory storingcomputer program instructions; and a processor configured to execute thecomputer program instructions to cause the apparatus at least to performthe method of any of claims 1 to
 19. 21. A computer program that, whenrun on a computer, performs the method of any of claims 1 to
 19. 22. Acarrier signal carrying the computer program as claimed in claim
 21. 23.A computer readable storage medium encoded with instructions that, whenexecuted by a processor, performs the method of any of claims 1 to 19.24. An apparatus for measuring oscillatory motion comprising: means forreceiving a plurality of signals related to a plurality of measurementsof a direction of a vector parameter; means for determining apredominant direction of the vector parameter based on the plurality ofmeasurements; and means for determining an angle of rotation between: ameasured direction of the vector parameter of one of the plurality ofmeasurements, and the predominant measured direction of the vectorparameter.
 25. An apparatus for measuring oscillatory motion comprising:an input interface arranged to receive a plurality of signals related toa plurality of measurements of a direction of a vector parameter; acontroller arranged to determine a predominant direction of the vectorparameter based on the plurality of measurements; and the controllerfurther arranged to determine an angle of rotation between: a measureddirection of the vector parameter of one of the plurality ofmeasurements, and the predominant measured direction of the vectorparameter.
 26. A system for measuring oscillatory motion comprising:means for measuring a direction of a vector parameter; means forgenerating a signal related to the direction of a vector parameter;means for transmitting the signal; and the apparatus according to any ofclaims 20 and 24 to
 25. 27. A device for measuring oscillatory motioncomprising: means for measuring a direction of a vector parameter; meansfor generating a signal related to the direction of a vector parameter;means for transmitting the signal; and the apparatus according to any ofclaims 20 and 24 to
 25. 28. A system as claimed in claim 26 or a deviceas claimed in claim 27, wherein the means for measuring the direction ofthe vector parameter is arranged to be coupled to a body part.
 29. Asystem as claimed in claim 26 or a device as claimed in claim 27,wherein the means for measuring the direction of the vector parametercomprises at least one of: means for measuring a gravitational field,means for measuring a magnetic field, and means for measuring angularorientation.
 30. A system as claimed in claim 26 or a device as claimedin claim 27, wherein the means for measuring the direction of the vectorparameter is arranged to measure the direction of the vector parameterin three dimensions.
 31. A method, apparatus, computer program, systemand device for measuring oscillatory motion substantially ashereinbefore described with reference to and/or as shown in theaccompanying drawings.